THE PHYSICAL PKOPEETIES OF SOILS 



83 



depth of from 2 to 3 feet. The monthly and seasonal 

 variations extend to a greater depth, but a point is 

 ultimately reached at which these also are too small to 

 be recorded. The depth to which the variations extend 

 depends mainly upon the extent of the variations at the 

 surface. 



During the winter months the temperature of the soil is 

 generally rather higher than that of the air; in summer 

 it is generally somewhat lower. The differences become 

 more marked as the depth increases, as will be seen from 

 the following figures : 



MISCELLANEOUS PROPERTIES. 



Colour. Of the fundamental substances which make up 

 the bulk of the soil, organic matter alone is brown or black. 

 Silica, kaolin and carbonate of lime, when perfectly pure, 

 are white. Quartz crystals are often coloured by the pre- 

 sence of traces of metallic oxides chiefly oxides of iron. 

 Many of the common rock-forming minerals, e.g., felspars, 

 etc., are white or greyish white in the powdered condition. 

 Some of them, however especially those which contain 

 compounds of iron in larger or smaller proportion are 

 black or dark coloured. Magnetite and biotite are black ; 

 hornblende is usually almost black with a greenish or some- 

 times a reddish tinge ; augite and olivine are green ; 

 haematite is red and pyrites a golden yellow colour. The 



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